In recent years, the amount of information demanded is increasing, and therefore, in the field of information equipment, audiovisual equipment, and the like, attention is being given to an information recording medium, such as an optical disk, which is designed to have easy accessibility to data and a large storage capacity of data and contribute to downsizing of equipment in order to achieve higher-density information recording. For example, in the field of optical disks, there is proposed an optical recording medium having a storage capacity of about 25 GB an a single layer or about 50 GB in a dual layer under conditions where the wavelength of laser light is about 400 nm and the numerical aperture (NA) of a converging lens of a reproducing head for converging the laser light is 0.85, as described in Japanese Patent Laid-open Publication No. 2002-92969.
Hereinbelow, the structure of conventional multilayer information recording media described in Japanese Patent Laid-open Publication No. 2002-92969 and production methods thereof will be described with reference to FIGS. 2 to 4.
FIGS. 2(a) to 2(f) illustrate a method for producing a stamper that is a substrate production die to be used for producing a conventional multilayer information recording medium. The stamper is produced in the following manner. First, a photosensitive material, such as a photoresist, is applied onto a glass plate 201 to form a photosensitive film 202 (see FIG. 2(a)). Then, the photosensitive film 202 is exposed to laser light 203 to transfer a pattern of pits and/or guide grooves thereto (see FIG. 2(b)). As a result of this exposure, as shown in FIG. 2(b), a light-exposed portion 202a is formed. Then, the photosensitive film 202 is subjected to a development process to remove the photosensitive material of the light-exposed portion 202a. As a result, an optical recording master 205 having a pattern of pits and/or guide grooves 204 is produced (see FIG. 2(c)). On this pattern of pits and/or guide grooves 204 of the optical recording master 205, a conductive film 206 is formed by sputtering, vapor deposition, or the like. In this way, the shape of the pattern of pits and/or guide grooves 204 formed in the photosensitive film 202 is transferred to the conductive film 206 (see FIG. 2(d)). Further, a plating film 207 is formed on the conductive film 206 to increase the rigidity and thickness of the conductive film 206 (see FIG. 2(e)). Then, the conductive film 206 having the plating film 207 formed thereon is separated from the optical recording master 205 along the interface between the photosensitive film 202 and the conductive film 206 to obtain a stamper 208 (see FIG. 2(f)).
FIG. 3 is a cross-sectional view of the conventional multilayer information recording medium. The multilayer information recording medium includes a first signal substrate 301, a first thin film layer 302, a second signal substrate 303, a second thin film layer 304, and a transparent substrate 306, and a transparent layer 305. The first signal substrate 301 has a signal surface provided with pits and/or guide grooves formed by transfer, and the first thin film layer 302 is laminated on the signal surface of the first signal substrate 301. The second signal substrate 303 has a signal surface provided with pits and/or guide grooves formed by transfer on the opposite side of a surface thereof bonded to the first thin film layer 302. The second thin film layer 304 is laminated on the signal surface of the second signal substrate 303. The transparent substrate 306 is provided so as to be opposed to the second signal substrate 303. The transparent layer 305 is provided to laminate the second thin film layer 304 and the transparent substrate 306 together.
As described above, the first signal substrate 301 has a signal surface provided with pits and/or guide grooves formed by transfer. Such a signal surface is formed using the stamper 208 shown in FIG. 2(f) when the first signal substrate 301 is produced by injection compression molding or the like. On the thus formed signal surface, a thin film layer is laminated. Such a thin film layer laminated on the signal surface is referred to as an information recording layer. It is to be noted that a thickness of the first signal substrate 301 is about 1.1 mm.
The first thin film layer 302 includes a recording film and/or a reflective film, and is formed by sputtering, vapor deposition, or the like on the signal surface of the first signal substrate 301 provided with pits and/or guide grooves.
The second signal substrate 303 is formed in the following manner. First, the first signal substrate 301 having the first thin film layer 302 laminated thereon is spin-coated with a photo-curing resin to form a photo-curing resin layer, and then a signal transfer substrate having a signal surface provided with pits and/or guide grooves is laminated on the photo-curing resin layer so that the signal surface of the signal transfer substrate faces toward the first signal substrate 301. The photo-curing resin is cured by irradiation with light, and then the signal transfer substrate is separated from the first signal substrate 301 along the interface between the signal transfer substrate and the photo-curing resin layer to obtain a second signal substrate as a resin layer having a signal surface. The second thin film layer 304 is formed on the second signal substrate 303 in the same manner as in the case of forming a first thin film layer 302.
Finally, the transparent substrate 306 is formed using a material transparent to recording/reproducing light (i.e., a material having optical transparency) so as to have a thickness of about 0.1 mm. Between the transparent substrate 306 and a laminated substrate 307, there is provided the transparent layer 305 to bond them together. The transparent layer 305 is formed of a photo-curing resin or an adhesive such as a pressure-sensitive adhesive.
The recording/reproducing of information onto/from the thus formed multilayer information recording medium is performed by allowing recording/reproducing laser light to enter the multilayer information recording medium from the transparent substrate 306 side thereof.
FIGS. 4(a) to 4(j) illustrate a method for producing another conventional multilayer information recording medium different from the above-described one. This production method will be described with reference to FIGS. 4(a) to 4(j).
First, a first signal substrate 401 is fixed to a turntable 403 by means of vacuum suction or the like (see FIG. 4(a)). Then, a first thin film layer 402 containing a recording film material and/or a reflective film material is laminated by sputtering, vapor deposition, or the like on a first signal surface of the first signal substrate 401 provided with pits and/or guide grooves to form a first information recording layer. Onto the first thin film layer 402 formed on the first signal substrate 401 fixed to the turntable 403, a photo-curing resin A 404 is fed by a dispenser in such a manner as to form a circle concentric with the first signal substrate at the inner radius of the first signal substrate (see FIG. 4(b)). Then, the first thin film layer 402 is coated with the photo-curing resin A 404 by spinning the turntable 403. At this time, the excess resin and air bubbles are removed from the photo-curing resin A 404 by centrifugal force. In addition, the thickness of the coating of the photo-curing resin A 404 can be adjusted to a desired value by appropriately setting the viscosity of the photo-curing resin A 404, the number of revolutions of the turntable 403, the rotation time of the turntable 403, ambient conditions (e.g., temperature, humidity) around the turntable 403, and the like. After the rotation of the turntable 403 is stopped, the coating of the photo-curing resin A 404 is cured by irradiation with light emitted from a light irradiator 405 (see FIG. 4(c)).
Then, a second information recording surface is formed on the first signal substrate 401 in the following manner. A transfer substrate 406 having a signal surface provided with pits and/or guide grooves is fixed to a turntable 407 (see FIG. 4(d)). Onto the transfer substrate 406 fixed to the turntable 407, a photo-curing resin B 408 is fed by a dispenser in such a manner as to form a circle concentric with the transfer substrate 406 at the inner radius of the transfer substrate 406 (see FIG. 4(e)). Then, the transfer substrate 406 is coated with the photo-curing resin B 408 by spinning the turntable 407. As in the case of coating with the photo-curing resin A404, the thickness of the coating of the photo-curing resin B 408 can be adjusted to a desired value. After the rotation of the turntable 407 is stopped, the coating of the photo-curing resin B 408 is cured by irradiation with light emitted from a light irradiator 409 (see FIG. 4(f)).
The thus obtained two substrates 410 and 411 are laminated together on the turntable 403 with a photo-curing resin C 412 being interposed therebetween so that their respective cured resin layers 404 and 408 are opposed to each other (see FIG. 4(g)). The substrates 410 and 411 laminated together in such a manner as described above is spun on the turntable 403 so that the thickness of the photo-curing resin C 412 is adjusted to a desired value. Then, the photo-curing resin C 412 is cured by irradiation with light emitted from the light irradiator 405 (see FIG. 4(h)). After the substrates 410 and 411 are bonded together with the photo-curing resin C 412, the transfer substrate 406 is separated from the substrate 410 along the interface between the transfer substrate 406 and the cured resin layer 408. In this way, a second signal surface is formed on the first signal substrate 401.
It is to be noted that the photo-curing resin A 404 is selected from resins having good adhesion to the first thin film layer 402 and the photo-curing resin C 412, and the photo-curing resin B 408 is selected from resins having good releasability from the transfer substrate 405 and good adhesion to the photo-curing resin C 412. These photo-curing resins A, B, and C may be the same or different from each other. In addition, the photo-curing resins A, B, and C each have a viscosity of about 0.15 Pa·s to make the thickness of their respective resin layers as small as possible.
Subsequently, on the second signal surface formed on the first signal substrate 401, a second thin film layer 413 containing a recording film material and/or a reflective film material is formed by sputtering, vapor deposition, or the like. The second thin film layer 413 serves as a second information recording layer. Finally, a transparent layer 415, which is substantially transparent to recording/reproducing light (i.e., which substantially transmits recording/reproducing light), is formed to bond the second thin film layer 413 and a transparent substrate 414 together. Such a transparent layer 415 is formed in the following manner. A photo-curing resin is dropped onto the second thin film layer 413. After the second thin film layer 413 is coated with the photo-curing resin by spinning the turntable 403, air bubbles contained in the photo-curing resin is removed and the thickness of the coating of the photo-curing resin is controlled. Then, the photo-curing resin is cured by irradiation with light to form a transparent layer 415 (see FIG. 4(i)).